Electrodes have been formed on zirconia solid electrolyte bodies by both thick film techniques and thin film techniques, as described in U.S. Pat. No. 4,169,778 which was issued to G. S. Mann, M. P. Murphy, D. R. Fredericks and K. R. Deming. The solid electrolyte for that sensor is in the shape of a tapered cup having inner and outer electrodes. The inner electrode can be a platinum coating applied by sintering the platinum to the zirconia, without using any appreciable bonding agent. Such a coating is highly porous but is usually not desirable for direct contact with a moving exhaust gas stream. Porous platinum coatings can be produced by sputtering techniques but the coatings are so thin that they can be abraded away by the moving exhaust gas stream. Consequently, it is known to use a porous overcoat to isolate the exhaust gas electrode from direct impingement of a moving exhaust gas stream. The sputtering of thin film platinum electrodes and the overcoating of them with a ceramic layer is described in United States patent application Ser. No. 89,264, entitled "Exhaust Electrode Process for Exhaust Gas Oxygen Sensor", filed Oct. 29, 1979 in the names of T. J. Gold, F. L. Kennard, III, P. C. Kikuchi and R. V. Wilhelm, Jr., and assigned to the assignee of this invention. Such techniques require costly equipment and a costly manufacturing environment.
Electrodes can also be made of coatings comprising powdered platinum and glass. I refer to these latter coatings as cermets. Cermets will bond well to the zirconia surface and will be quite durable with respect to erosion from the aforementioned exhaust gas stream. On the other hand, cermet coatings are not extremely porous and generally do not provide fast responding sensors.
I have found how to make a highly porous cermet coating useful as an exhaust gas electrode on a zirconia-type exhaust gas oxygen sensor. After aging, the sensors are fairly fast, e.g. less than 200 milliseconds in response time. Also, testing to the equivalent of over 40,000 km did not produce extensive electrode wear or cracking. Durability is thus apparently quite high. I further believe that my electrode is not only distinctive in its porous structure but in the manner in which it is made.